As is well known, lead acid batteries can be produced in which each electrode consists of one or more plates, each plate being composed of a laminar grid surrounded by active material. By pasting the appropriate active material onto the grids, both the positive and the negative electrodes can be produced as stacks of such laminar plates. Within each plate the grid serves as a support for the active material as well as a conductor of electricity to and from the active material. The grid is a metallic structure made of lead or a lead alloy containing small amounts of such metals as calcium, antimony or tin. The many factors which influence the design of a battery grid impose constraints which are mutually inconsistent so that designs invariably represent compromises aimed at achieving an optimum combination of characteristics. Some important design-influencing factors are:
(a) minimizing the grid weight;
(b) minimizing the internal resistance of the grid;
(c) ensuring ease of fabricability of the grid;
(d) ensuring the ability to support the required amount of active material; and
(e) providing compartments of adequate dimensions to ensure the desired size of the active material "biscuits" contained therein.
A basic design for laminar grids includes a rectangular frame having a lug at or near a corner thereof, to constitute the current carrying connection of the plate, and a plurality of wires arranged orthogonally as "verticals" or "horizontals" to divide the space within the frame into discrete rectangular pockets. (The terms vertical and horizontal are used here with reference to the orientation of the wires when the grid is mounted in a battery, i.e., with the lug uppermost). Because higher currents are carried by wires near the lug, attempts have been made to improve the resistance characteristics of such grids by use of tapering wires, increased number of verticals and use of some diagonal wires. One such grid is described in U.S. Pat. No. 3,989,539.
A different approach to improving the efficiency of grids, and one which is finding more and more favor in the art, involves adopting grids designs wherein the pattern of wires within the frame is not orthogonal. U.S. Pat. No. 3,452,145 describes grids wherein a first set of wires are disposed along the lines of radii emanating from the vicinity of the lug, while the second set of lines lie along arcs centered in the vicinity of the lug. A variation of this "radial" type of configuration is described in U.S. Pat. No. 3,690,950 wherein lightness of weight is achieved by using a frame and cross wires made of plastic, together with a set of metallic fingers radiating from a lug location to two sides of the frame. The latter type of grid requires more elaborate methods of fabrication than an all-metal grid which is castable.
I am also aware of a commercially produced battery which incorporates an all-metal grid having a lug at one corner, a set of horizontal wires and a set of diverging wires connecting the lug-carrying top of the frame to the bottom edge or to the side edge remove from the lug. Such a design is illustrated schematically in FIG. 1 of the accompanying drawings.
For several reasons it is preferable to arrange for the lug of a grid to be substantially spaced from the grid corner. One reason for this is the improvement of performance, as judged by power output at high current discharge rate. Such performance is particularly important in automotive batteries wherein high cold-cranking available power is demanded. The off-setting of the lug is also beneficial to the overall design and method of construction of batteries using the grid. Thus as is well known, the assembly of a battery involves stacking together several grids, attaching a strap to their lugs and then positioning the stacks in the battery case with separating partitions therebetween. Connections then have to be made between pairs of straps separated by a partition. In the case of grids having lugs located at their corner, the straps to be interconnected are close to the battery casing. For engineering reasons, it has generally been necessary to resort to "risers" which are offset relative to the straps so that they are sufficiently spaced from the casing. On the other hand, where lugs are themselves offset relative to the grid corner, the straps to be interconnected in the battery are sufficiently spaced from the casing to allow for a rectilinear connection through the partition which not only saves cost and weight, but also minimizes internal resistance.
If attempts are made to cast grids of the design illustrated in FIG. 1 but having lugs offset relative to the grid corner, problems of lead flow are encountered which give rise to porosity in the lug region of the frame. This is because a relatively large amount of lead has to flow from the fill edge to the lug region; grids are almost invariably cast in pairs with the fill edge being the side edge nearer to the lug, i.e, the left edge as viewed in FIG 1. The result may be an unacceptably high rate of rejection of the castings.